1. Field of the Invention
This invention relates to a dimmer control circuit for regulating the luminous intensity of a lamp provided on an operation panel for a vehicle.
2. Description of the Prior Art
A conventional dimmer control circuit having time constant regulating means and a free-running multivibrator for controlling the luminous intensity of a lamp by regulating the time constant regulating means is heretofore used as constructed in FIG. 2.
As shown in FIG. 2, a resistor 2 is connected at the input side of a digital amplifier 1 between a bias terminal t.sub.1 and a signal terminal t.sub.2, and a transistor 3 is connected at its collector to the signal terminal t.sub.2 and at its emitter to an earth terminal t.sub.3. A predetermined voltage is applied to the bias terminal t.sub.1, and the earth terminal t.sub.3 is grounded. The regulating terminal of a variable resistor 4 is connected to the bias terminal t.sub.1, and the resistor 4 is connected at both ends through resistors 5 and 6 to the one electrodes of capacitors 7 and 8.
Further, the other electrode of the capacitor 7 is connected to the signal terminal t.sub.2, and the other electrode of the capacitor 8 is connected through a resistor 9 to the bias terminal t.sub.1. The anode of a diode 10 is connected to the connecting point of the capacitor 7 to the resistor 5, the cathode of the diode 10 is connected to the base of a transistor 11, the emitter of the transistor 11 is connected to the earth terminal t.sub.3, and the collector is connected to the connecting point of the capacitor 8 and the resistor 9.
Similarly, the connecting point of the capacitor 8 to the resistor 6 is connected to the anode of a diode 12, and the cathode of the diode 12 is connected to the base of the transistor 3.
A lamp 13 is connected at the output side of the digital amplifier 1 in FIG. 2 between a bias terminal t.sub.4 and a signal terminal t.sub.5, and a battery 14 is connected between the bias terminal t.sub.4 and an earth terminal t.sub.6.
As described above, a free-running multivibrator 15 is located at the input side of the digital amplifier 1, the variable resistor 4 is provided as time constant regulating means, the low voltage and current of the input side are amplified by the digital amplifier 1, the battery 14 to the lamp 13 is controlled ON or OFF at the output side to regulate the luminous intensity of the lamp 13.
In the conventional dimmer control circuit constructed as described above, when the resistance value of the variable resistor 4 is varied, the load time ratio (duty ratio) of the ratio of the pulse continuation time to the pulse time of the output pulse signal of the multivibrator 15 is varied.
The variation of the load time ratio causes the voltage of the battery 14 applied to the lamp 13 to be controlled ON or OFF by the digital amplifier 1, and when the load time ratio is reduced, the ON time of the voltage of the battery 14 is increased in response to the reduction of the load time ratio, thereby increasing the luminous intensity of the lamp 13.
When the load time ratio is increased, the ON time of the voltage of the battery 14 is decreased in response to the increase in the load time ratio, thereby reducing the luminous intensity of the lamp 13.
In this manner, in the conventional dimmer control circuit described above, the luminous intensity of the lamp 13 can be controlled by varying the resistance value of the variable resistor 14.
In the conventional dimmer control circuit described above, when the variable regulator 4 is set to a position for maximum luminous intensity of the lamp 13, the multivibrator 15 remains unstable during the maximum luminous intensity of the lamp 13.
Consequently, during maximum luminous intensity of the lamp 13, since a positive output pulse signal of narrow pulse width is outputted from the multivibrator 15, the luminous intensity of the lamp 13 is finely flickered, and wasteful power is consumed at the input side of the digital amplifier 1.
To prevent this flickering, a dimmer control circuit of a configuration as shown in FIG. 3 has been proposed.
In the dimmer control circuit proposed as described above, where the same numerals as those in FIG. 2 denote the same or equivalent components in FIG. 3, a switch 16 is provided between a signal terminal t.sub.5 and an earth terminal t.sub.6 at the output side of a digital amplifier 1, and the switch 16 is closed through the digital amplifier 1 near the end of a regulating direction for increasing the luminous intensity of the lamp 13 of the variable resistor 4.
Thus, in the dimmer control circuit proposed as described above, when the resistance value of the variable resistor 14 is regulated in a direction for increasing the luminous intensity of the lamp 13, the switch 16 is closed near the end of the regulation that the luminous intensity of the lamp 13 becomes maximum. Therefore, the voltage of the battery 14 is directly applied to the lamp 13 so that the lamp 13 becomes maximum in its luminous intensity.
Thus, in the dimmer control circuit proposed as described above, the lamp 13 can irradiate stable illumination light without fine flicker in the maximum luminous intensity state, but since a relatively large current flows at the output side of the digital amplifier 1, the switch 16 needs the contact capacity of approx. 3 to 10 A for controlling the current of the lamp 13.
Wasteful power is consumed at the input side of the digital amplifier 1 due to the irregularity in the circuit components of the multivibrator 15 and the digital amplifier 1 or the variation in the operating conditions even in the maximum luminous intensity state of the lamp 13.